Method of dewaxing shell molds

ABSTRACT

Refractory shell molds are made by a method including the steps of forming a shell investment mold over a wax pattern, freezing the mold and pattern to a temperature in the vicinity of about 100* F. whereby the pattern shrinks more than the mold to provide a slight gap between the pattern and the mold and then immediately placing the mold and pattern in a heated environment whereby the pattern is removed by melting without cracking or otherwise damaging the mold.

United States Patent 11 1 Heath Nov. 6, 1973 METHOD OF DEWAXING SHELLMOLDS [75] Inventor: Russel V. Heath, Indianapolis, Ind.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

1221 Filed: Feb. 7, 1972 21 Appl. No.: 224,328

52 US. (:1. 164/35 51 Int. Cl. 1322c 9/04 [58] Field of Search 164/34,35, 338 M [56] References Cited UNITED STATES PATENTS 5/1970 Phelps.164/34 X FOREIGN PATENTS QR APPLICATIONS Great Britain 164/35 PrimaryExaminerJ. Spencer Overholser Assistant Examiner-John E. RocthelAtt0rneyPeter P. Kozak et a1.

[57] ABSTRACT 3 Claims, No Drawings 1 METHOD OF DEWAXING SHELL MOLDSBACKGROUND OF THE INVENTION The invention herein described was made inthe course of work under contract or subcontract thereunder with theUnited States Air Force.

This invention relates to the art of precision casting of metal articlesand particularly to a method for removing patterns from shell or thinwalled one-piece molds of ceramic refractory materials.

As is well known a major problem in the art of making thin walledinvestment molds is the task of removing the wax pattern from the moldprior to firing or sintering it without cracking or otherwise damagingthe mold. The problem results from the fact that when the pattern isremoved bymelting it the wax undergoes a marked expansion before itliquifies so that the wax pattern exerts very high pressures against themold walls sufficient to crack or otherwise damage them. Varioustechniques have been developed for dealing with this problem. One suchmethod proposes to make a thinly invested shell mold by utilizing frozenmercury for the pattern which has very small thermal coating ofexpansion near its melting point so that it can be melted and removedwithout damaging the mold. However, the use of a frozen mercury patternrequires carrying the various coating steps for making the mold at atemperature belowv "39 C and requires the use of a non aqueous slip orone having a suitably low freezing point. Such temperature limitationscause the process to be relatively expensive and difficult to perform.

Another method which has been proposed involves dipping the pattern andmold into a liquid maintained at a temperature equal to or above themelting point of the pattern material. This causes the hot liquid to beforced under pressure into the porous walls of the mold to therebycontact the pattern at the interface between the pattern and the moldwhich softens the pattern material at its surface before the body of thepattern has been heated high enough for it to exert expansion stresseson the mold sufficient to break it.'This method, however, hasshortcomings in that the liquid tends to weaken the'mold structure andcauses undesirable discontinuities in the casting surface of the mold.

In another method it has been proposed that the mold and pattern besubjected to solvent vapors for the wax pattern material. This methodhas disadvantages in that such vapors are usually toxic and combustible.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod for making refractory shell molds of the type having walls thinenough to allow rapid transfer of heat therethrough and not sufficientlythick to resist breakage thereof from the force which would result fromexpanison of a wax pattern contained therein by which the wax pattern isremoved from the mold prior to firing or sintering simply andefficiently without damage to the mold. A more specific object is toprovide a method of making refractory shell molds in which the mold andthe wax pattern contained therein are frozen whereby 1 the patterncontracts sufficiently more than the mold to provide a sufficient gaptherebetween so that when the mold and pattern assembly is inserted in aheated environment substantially above the melting temperature of thewax the heat penetrates rapidly through the mold to cause the mold toexpand more rapidly than the pat tern and meet the surface layer of thepattern, and the gap between the pattern and mold is sufficient toeliminate any internal pressure build-up due to wax expansion whichwould damage the mold.

These and other objects are carried out by forming refractory shellmolds over the wax pattern by any suitable method as, for example,sequentially dipping the pattern a plurality of times in suitableceramic shell dip slurries with intermediate drying of the dip coat,then placing the dried molds in a cold box maintained at about -80 F toabout -l00 F for a time sufficient to cause the. mold and pattern toapproach the box temperature, and then removing the molds from the coldbox and placing them directly in a heated environment operative to causeheat to penetrate rapidly through the mold for a time sufficient toinitially melt a surface layer of the wax pattern and then to completelyremove the wax pattern by melting it.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of this inventionincludes first forming a suitable ceramic or refractory shell mold abouta heat destructible pattern. The refractory mold material may be anysuitable composition applied by any suitable method and the pattern mayconveniently be formed of wax. A suitable mold composition and method ofapplication is described hereinafter.

Three different ceramic dip slurries are prepared. The first or primedip consists of 200 lbs. of milled zircon (zirconium silicate) mixedwith 48 lbs. of a 30 percent by weight aqueous-solution of colloidalsilica and 30 ccs of a wetting agent such as an alkyl aryl sodiumsulphonate. A second dip is prepared consisting of 200 lbs. of milledzircon with 38 lbs. of the 30 percent aqueous colloidal silica. A thirdor backup slurry is prepared consisting of 150 lbs. of fused silicamixed with 78 lbs. of the 30 percent colloidal silica. These dips aremaintained at a temperature of F and the humidity environment of 60percent. The refractory coating is applied in a sequence of steps bywhich the pattern is alternately dipped in a slurry bath and dried in asteam autoclave under controlled temperature and humidity conditions.

First, a wax pattern is exposed to an environment having a dry bulbtemperature of 84 F and a wet bulb temperatuie of 73 F to condition thepattern.

Second, the pattern is dipped in the first slurry dip and stuccoed in afluidized bed of 80 mesh zircon and dried for about 15 minutes in anautoclave maintained at 84 F dry bulb temperature and 73 F wet bulbtemperature.

Third, the pattern is dipped in the second bath, stuccoed in the 80 meshzircon fluidized bed and dried in the autoclave adjusted to a F dry bulbtemperature and a 74 F wet bulb temperature.

Fourth, the pattern is dipped in the third or back-up dip, stuccoed in a50 mesh silica fluidized bed and dried in the autoclave adjusted to a 96F dry bulb temperature and a 77 F wet bulb temperature.

Fifth, the pattern is dipped in the third back-up slurry, stuccoed inthe 50 mesh silica fluidized bed and dried for about 15 minutes in theautoclave adjusted to a 100 F dry bulb temperature and a 75 F wet bulbtemperature.

Sixth, the pattern is dipped in the back-up slurry, stuccoed in the 50mesh silica fluidized bed and dried in the autoclave adjusted to a 106 Fdry bulb temperature and a 72 F wet bulb temperature.

Seventh, the pattern is dipped in the back-up slurry, stuccoed in the 50mesh silica fluidized bed and dried for about 15 minutes in theautoclave adjusted to the 106 F dry bulb temperature and 72 F wet bulbtemperature.

Eight, the pattern is dipped in the back-up slurry and allowed to dryfor about 15 minutes in the autoclave at the 106 F dry bulb temperatureand 72 F wet bulb temperature.

The resulting shell mold and pattern is then placed in a cold boxmaintained at about -100 F and held therein until the pattern and moldreaches a temperature ofl F which is a period of about 20 minutes. Underthis temperature a gap of about 0.002 inch is formed between the patternand the mold. The pattern and mold are then removed from the cold boxand immediately placed in an autoclave maintained at a temperature of230 F with a steam pressure of 80 psi gauge. In the autoclave the heatreadily penetrates the thin shell mold and melts a surface layer of thewax pattern before the pattern as a whole warms up sufficiently. Theexpansion due to the melting of the surface layer is taken up by the gapso that the wax pattern is melted out without any damage to the mold.Thereafter the mold is fired in a gas fired furnace at about 1800 F forabout 30 minutes to produce a mold suitable for use in casting metals.Firing temperatures of l600 F to 2000 F are useable and temperatures of1800 F to 2000 F are preferred for best results.

It has been found that a temperature of 80 F or less will produce a gapbetween the pattern and the mold in the vicinity of .002 inch which issufficient to prevent the melting surface layer of the wax pattern fromsubjecting the mold to damaging pressures. Temperatures in the vicinityof 80 F may be obtained in the cold box by the use of dry ice andacetone as the freezing medium. Preferably temperatures of 100 F areused which are obtainable by the use of liquid nitrogen. Temperaturesbelow l00 F do not appear advantageous. Moreover, temperaturessignificantly less than 100 F are not desired since deleterious effectssuch as cracking of the mold may be experienced.

Alternatively after the freezing step described above, the wax patternmay be removed by placing the mold and wax assembly directly in a gasfired furnace at about l600 F. The high heat is considered necessary toobtain a fast penentration of the heat through the mold to the waxsurface so that a surface layer of the wax is melted before the waxpattern as a whole is heated appreciably. Although the mold and patternassembly could be fired directly after freezing at the usual firingtemperatures, this is not a preferred procedure because the wax tends toburn and may have adverse effects on the mold surface. The frozen moldand wax pattern assembly may also be immersed in a wax bath maintainedat about 450 F to remove the wax pattern.

However, this method is not preferred because of residual wax left onthe mold surfaces.

It will be apparent to those skilled in the art that various refractoryshell mold compositions and various methods of application to a waxpattern may be used and various heating methods may be employed toremove the wax after freezing within the scope of this invention.

It is claimed:

1. The method of removing a wax pattern from a shell investment moldformed thereon, said mold having a wall thin enough to allow rapidtransfer of heat therethrough and not sufficiently thick to resistbreakage from the force which would result from the expansion of saidwax pattern upon slow heating thereof, which comprises freezing saidmold and pattern contained therein to a temperature of at least F tocreate a gap between the mold and the pattern, and then immediatelyheating the mold and pattern in a manner such that the heat rapidlypenetrates the mold wall and initially melts a surface layer of the waxpattern which expands into said gap so that the expansion forces are notsufficient to damage the mold and then completely melts the wax patternto remove it from the mold.

2. The method of removing a wax pattern from a shell investment moldformed thereon, said mold having a wall thin enough to allow rapidtransfer of heat therethrough and not sufficiently thick to resistbreakage from the force which would result from the expansion of saidwax pattern upon slow heating thereof, which comprises freezing saidmold and pattern contained therein to a temperature of about 80 F toabout F to create a gap between the mold and the pattern of about 0.002inch and then immediately heating the mold and pattern in a manner suchthat the heat rapidly penetrates the mold wall and initially melts asurface layer of the wax pattern which expands into said gap so that theexpansion forces are not sufficient to damage the mold and thencompletely melts the wax pattern to remove it from the mold.

3. The method of removing a wax pattern from a shell investment moldformed thereon, said mold having a wall thin enough to allow rapidtransfer of heat therethrough and not sufficiently thick to resistbreakage from the force which would result from the expansion of saidwax pattern upon slow heating thereof, which comprises freezing saidmold and pattern contained therein to a temperature of about 80 F toabout 100 F to create a gap between the mold and the pattern of about0.002 inch and then immediately heating the mold and pattern in a steamautoclave maintained at about 230 F at about 80 psi gauge so that theheat rapidly penetrates the mold wall and initially melts a surfacelayer of the wax pattern which expands into said gap so that theexpansion forces are not sufficient to damage the mold and thencompletely melts the wax pattern to remove it from the mold.

* III

1. The method of removing a wax pattern from a shell investment moldformed thereon, said mold having a wall thin enough to allow rapidtransfer of heat therethrough and not sufficiently thick to resistbreakage from the force which would result from the expansion of saidwax pattern upon slow heating thereof, which comprises freezing saidmold and pattern contained therein to a temperature of at lEast -80* Fto create a gap between the mold and the pattern, and then immediatelyheating the mold and pattern in a manner such that the heat rapidlypenetrates the mold wall and initially melts a surface layer of the waxpattern which expands into said gap so that the expansion forces are notsufficient to damage the mold and then completely melts the wax patternto remove it from the mold.
 2. The method of removing a wax pattern froma shell investment mold formed thereon, said mold having a wall thinenough to allow rapid transfer of heat therethrough and not sufficientlythick to resist breakage from the force which would result from theexpansion of said wax pattern upon slow heating thereof, which comprisesfreezing said mold and pattern contained therein to a temperature ofabout -80* F to about -100* F to create a gap between the mold and thepattern of about 0.002 inch and then immediately heating the mold andpattern in a manner such that the heat rapidly penetrates the mold walland initially melts a surface layer of the wax pattern which expandsinto said gap so that the expansion forces are not sufficient to damagethe mold and then completely melts the wax pattern to remove it from themold.
 3. The method of removing a wax pattern from a shell investmentmold formed thereon, said mold having a wall thin enough to allow rapidtransfer of heat therethrough and not sufficiently thick to resistbreakage from the force which would result from the expansion of saidwax pattern upon slow heating thereof, which comprises freezing saidmold and pattern contained therein to a temperature of about -80* F toabout -100* F to create a gap between the mold and the pattern of about0.002 inch and then immediately heating the mold and pattern in a steamautoclave maintained at about 230* F at about 80 psi gauge so that theheat rapidly penetrates the mold wall and initially melts a surfacelayer of the wax pattern which expands into said gap so that theexpansion forces are not sufficient to damage the mold and thencompletely melts the wax pattern to remove it from the mold.